This invention relates to apparatus for and a method of fabricating a Coriolis flowmeter and more particularly a Coriolis flowmeter formed primarily of plastic.
Coriolis flowmeters are in widespread use in applications that require the generation of accurate information regarding material flow. This information includes mass flow rate and material density. Coriolis flowmeters range in size from meters having a flow tube 0.16 centimeters in diameter to those 15 centimeters in diameter. These flowmeters serve wide range of material flows ranging from approximately several drops per minute, such as for use in anesthesiology systems, to several tons a minute, such as for use in oil pipelines or the loading and unloading of oil tankers. Regardless of its size, most of the applications in which Coriolis flowmeters are used require the highest degree of accuracy such as, for example, a maximum error of 0.15 percent. Also, many of the applications in which Coriolis flowmeters are used involve the generation of flow information for material that is hazardous and for which great care must be taken to prevent material leaks into the environment.
It is a problem that these stringent requirements have heretofore resulted in a high cost of manufacture of Coriolis flowmeters. This high cost of manufacture results from the expensive materials that must be used, such as stainless steel and titanium. This high cost of manufacture also results from the complexities of the manufacturing processes currently used to produce high quality Coriolis flowmeters meeting the above discussed requirements. These steps include extensive machining, welding, brazing, and assembly of parts. Another requirement is that flow tubes of curved flow tube meters must have a constant curvature and be free from kinks. These requirements increase the complexity of the machining and bending operations required to fabricate the flow tube.
Another problem is with the brazing operations used to join the various flowmeter elements. Braze joints are typically used to affix the flow tube to the brace bar. Braze joints are also used to join other parts such as driver and pick off brackets and to affix a manifold to the ends of U shaped flow tubes. Considerable care must be taken in the brazing operations to produce braze joints that securely affixes elements to one another and that are free from microscopic cracks. Also, the brazing operation generates thermal stresses in which a brace bar can cool faster than the flow tube or the other elements to which the brace bar is connected. This rapid and uneven cooling generates a permanent stress in the elements to which the brace bar is connected.
Another problem is that Coriolis flowmeters are not devices that are produced in volumes on an assembly line. They are low production quantity devices which are handcrafted and carefully inspected at each stage of the manufacturing process to ensure that each part meets its design specifications and is of the required accuracy before it is joined to another part. This high degree of care is required to ensure that the completed flowmeter meets its design specifications and is free from defects which could impair its output accuracy or cause its failure.
Another problem of Coriolis flowmeters is that they are often required to process corrosive materials. This degrades the life expectancy and reliability of the flowmeters unless they are fabricated using exotic materials such as stainless steel or titanium. These materials are expensive to purchase and are difficult to fabricate. The use of these materials often results in a flowmeter having elements formed of dissimilar materials; such as a flowmeter that has some stainless steel elements that must be joined to a titanium flow tube to provide an all titanium material flow path that is highly resistant to corrosive process materials.
Another problem of Coriolis flowmeters is that metal flow tubes of an acceptable thickness are relatively stiff and resistant to bending. The thicker the flow tube wallxe2x80x94the stiffer the flow tube. This stiffness opposes the Coriolis forces generated by the material flow and reduces the Coriolis deflections of the vibrating flow tube with material flow. This, in turn, reduces the flowmeter sensitivity by reducing the phase difference of the output signals generated by the flow tube pick offs. This is a particular problem in Coriolis flowmeters which must use flow tubes having thick walls for the containment of high pressure materials. Thus, the use of any metal flow tube is a compromise between the wall thickness required by pressure containment requirements and the flow sensitivity required of the flowmeter. U.S. Pat. No. 5,157,975 discloses a Coriolis flowmeter having a glass flow tube. However, it is brittle and does not solve the above mentioned problems of Coriolis flowmeters having metal flow tubes.
In accordance with the present invention a Coriolis flowmeter is provided that achieves an advance in the art and solves the above problems including the problem of high material costs and difficulty of manufacturing. The flowmeter of the present invention solves these problems by the use of plastic for most of the elements embodying the flowmeter. The flowmeter of the invention solves the above problems using manufacturing techniques which permit many embodiments of the invention to be formed by injection molding. All embodiments of the invention make extensive use of plastic and injection molding. In particular, all embodiments have a dynamically active structure that is formed entirely of plastic by injection molding.
In accordance with a first possible exemplary embodiment, a Coriolis flowmeter is provided having a single straight flow tube, a surrounding plastic balance bar concentric with the flow tube and a plastic brace bar that connects the ends of the balance bar with the flow tube. The entirety of the dynamically active structure (the flow tube, the balance bar and the brace bar) is formed of plastic by injection molding. The flow tube ends may be subsequently coupled to end flanges by appropriate bonding techniques.
In accordance with a second possible embodiment of the invention, the elements of the dynamically active structure as well as the end flanges are formed of plastic by injection molding. This second embodiment provides a plastic wetted flow path that extends through the entirety of the length of the flowmeter with the material flow extending serially from an inlet flange, through the flow tube to an outlet flange. This embodiment is advantageous in that the plastic wetted flow path eliminates problems of corrosion resulting from an interaction between the process material and metal flowmeter elements such as titanium, stainless steel and other metals. With the possible exception of a driver and pick offs, and case, the entirety of the flowmeter is formed of plastic by injection molding.
The above embodiment is formed by an injection molding process that comprises a first step of forming a flow path core mold having a cavity that defines the physical characteristics of the flow path within the flowmeter. The cavity within the flow path core mold is filled with a metal compound of fusible alloys containing bismuth, lead, tin, cadmium and indium. These alloys have a low melting point of approximately 47xc2x0 Centigrade. The injected metal is then allowed to cool to its solid state at which time the split halves of the mold are separated and the formed metal is removed. This metal defines, with precision, the material flow path of the flowmeter.
The second step of the process involves forming a wrapper mold having a cavity that defines the exterior of the flowmeter elements be formed. The formed low temperature metal flow path core is inserted into the wrapper mold which is then injected with the plastic that is used to form the exterior of the flowmeter elements. The plastic in the wrapper mold is allowed to cool and solidify following which the split halves of the wrapper mold are separated and the formed plastic flowmeter element is removed. The exterior of the formed plastic defines the desired external characteristics of the flowmeter element. The metal flow path core defining the flow path remains contained with the plastic structure formed by the wrapper molding process. This plastic structure defining the flow path is then heated to the temperature required to melt the low temperature metal flow path core. The low temperature metal melts and flows out of the plastic flowmeter element so that the resulting structure is a flowmeter element having exterior physical characteristics defined by the void within the wrapper mold and having an inner flow path defined by the flow path metal core formed by the flow path core mold.
The plastic flow elements formed by the above process are advantageous in that their external physical characteristics are formed with precision by the void within the wrapper mold. The flow element has an interior flow path formed with precision by the low temperature metal flow path core formed by the core mold. This process provides an idealized flow path having walls that are free from the defects and irregularities typical of the current casting processes associated with the fabrication of metal flow manifolds.
Another embodiment of the invention provides a Coriolis meter having a single curved flow tube formed of plastic. This flowmeter can be fabricated by an injection molding process similar to that above described for single straight tube flowmeters.
Another embodiment of the invention provides a Coriolis flowmeter having a pair of straight tubes connected between an inlet flange and an outlet flange. The pair of flow tubes comprises a dynamically balanced structure formed of plastic which may be fabricated by injection molding in a manner similar to that above described.
Another embodiment of the invention provides a Coriolis flowmeter having a pair of curved flow tubes comprising a dynamically balanced structure and connected between an inlet flange and an outlet flange. This flowmeter may be formed of plastic and fabricated in an manner similar to that above described.
In accordance with another embodiment of the invention, all of the above described flowmeters have brace bars formed of plastic and fabricated by plastic injection molding so as to comprise an integral unit with associated flow tubes.
In accordance with another embodiment of the invention, a single straight tube flowmeter includes an associated balance bar for dynamic balance. The balance bar may either be concentric with and surround its associated flow tube or, alternatively, may be a separate member parallel to and spaced apart from its associated flow tube but coupled to the flow tube by means of an associated brace bar.
All of the above described alternatives provide a Coriolis flowmeter that makes extensive use of plastic for its parts. Some of the embodiments use plastic only for the flow tube or tubes; others use plastic for the entirety of the dynamic structure comprising the flow tube or tubes, balance bar, and brace bar. Other embodiments employ plastic for the end flanges so that the Coriolis flowmeter provides a wetted material flow path entirely of plastic. The plastic part or parts of the flowmeters are formed by injection molding so that the part or parts of the flowmeter that employ plastic comprise a single integral plastic element.
The flowmeters of the present invention minimize corrosion problems by the use of plastic materials. These flowmeters are easier to manufacture and therefore have lower costs because of the use of plastic injection molding techniques. These flowmeters avoid the prior art problems of nonuniform wall thickness. These Coriolis flowmeters are further advantageous since the employment of the plastic injection molding provides a flow tube having a controlled wall thickness. If desired, the side wall of the flow tube bar may have an axial change in thickness in order to accomplish modal tuning. Also, auxiliary elements such as side ribs may be placed on the flow tube or the balance bar to control lateral vibration. The flow tube and the balance bar and the brace bar comprise an integral structure. This integral structure may also include flanges or alternatively the flanges may be affixed at a later time by means of adhesive bonding or plastic solvent welding. The case, if provided, may be either metal or plastic and if plastic may be permanently affixed to the remainder of the plastic elements of the flowmeter to provide a single integral unit formed primarily of plastic except for necessary metal elements such as the electrical conductors needed to operate the flowmeter. Also, a plastic junction box may be glued to the plastic flowmeter after inserting wires through it.
The molds are machined with precision to form flow paths having ideal bends with well controlled inner diameter and outer diameter measurements. Out of round problems of the flow tube flow path are avoided. Also avoided are the non smooth, rough or irregular inner walls of the flow tube. Corrosion problems are minimized by the use of plastic. Also avoided are the failure of brazed and welded joints typical of metal flowmeters together with the avoidance of the thermal problems associated with welding and brazing operations. Also, the meter has a low weight and is easily disposed of at the end of its useful life by recycling the plastic. The plastic flow tubes are more flexible than are metal flow tubes of the same thickness. This increases flowmeter sensitivity by enabling the plastic flow tube to have a greater Coriolis response for a given flow rate.
An aspect of the invention is:
A Coriolis flowmeter comprising:
flow tube means adapted to receive a material flow from a flowmeter input and to extend said material flow through said flow tube means to a flowmeter outlet;
a driver for vibrating said flow tube means;
pick off means coupled to said flow tube means for generating output signals representing Coriolis deflections of said vibrating flow tube means with material flow;
means responsive to said output signals generated by said pick offs for generating output information pertaining to said material flow; and
characterized by the flow tube means is formed of plastic to define a plastic wetted material flow path that extends through the entirety of the length of said flow tube means.
Another aspect is that the Coriolis flowmeter includes an inlet flange and an outlet flange coupled to ends of said flow tube means to define said flowmeter inlet and said flowmeter outlet.
Another aspect is that the Coriolis flowmeter is characterized in that said inlet flange and said outlet flange are formed of plastic.
Another aspect is that the Coriolis flowmeter is characterized in that said plastic wetted material flow path further includes said inlet flange and said outlet flange with said material flow extending serially through said inlet flange and said flow tube means and said outlet flange.
Another aspect is that the Coriolis flowmeter is characterized in that said wetted material flow path includes plastic flow tube stubs that connect said flow tube means to said inlet flange and to said outlet flange.
Another aspect is that the Coriolis flowmeter further includes a case enclosing said flow tube means and said driver and said pick off means.
Another aspect is that the Coriolis flowmeter is characterized in that said case is formed of plastic.
Another aspect is that the Coriolis flowmeter is characterized in that said flow tube means comprises a single flow tube.
Another aspect is that the Coriolis flowmeter further comprises:
a balance bar oriented parallel to said flow tube; and
brace bar means coupling said flow tube to end portions of said balance bar.
Another aspect is that the Coriolis flowmeter is characterized in that said balance bar is formed of plastic.
Another aspect is that the Coriolis flowmeter is characterized in that said balance bar and said brace bar means are formed of plastic.
Another aspect is that the Coriolis flowmeter is characterized in that:
said brace bar means comprises first and second brace bars coupling ends of said balance bar to said flow tube; and
a wall surface of said flow tube contains corrugations in a portion of said flow tube between said brace bars.
Another aspect is that the Coriolis flowmeter is characterized in that:
said plastic wetted flow path further includes a plastic inlet flange and a plastic outlet flange coupled to ends of said flow tube; and
that said balance bar and said brace bar means are formed of plastic.
Another aspect is that the Coriolis flowmeter is characterized in that said balance bar and said brace bar means and said flow tube are enclosed within a case to define an integral Coriolis flowmeter structure formed of plastic.
Another aspect is that the Coriolis flowmeter is characterized in that:
said balance bar and said brace bar means and said flow tube are enclosed within a case to define an integral Coriolis flowmeter structure formed of plastic;
a plastic case connect link means couples an inner wall of said case to ends of said balance bar and to said flow tube and to said brace bar means.
Another aspect is that the Coriolis flowmeter further includes plastic links positioned intermediate said flange means and said case connect link means and coupling said inner wall of said case to said flow tube.
Another aspect is that the Coriolis flowmeter is characterized in that said balance bar contains surface elements formed of plastic for facilitating the mounting of said driver and said pick off means to said balance bar.
Another aspect is that the Coriolis flowmeter is characterized in that said driver has a plastic bobbin integral with said balance bar and further has an electrically conductive coil on said bobbin.
Another aspect is that the Coriolis flowmeter is characterized in that said pick off means has a plastic bobbin integral with said balance bar and further has an electrically conductive coil on said bobbin.
Another aspect is that the Coriolis flowmeter is characterized in that said balance bar encloses said flow tube.
Another aspect is that the Coriolis flowmeter is characterized in that said balance bar is parallel to said flow tube and has a longitudinal axis offset from the longitudinal axis of said flow tube.
Another aspect is that the Coriolis flowmeter is characterized in that said flow tube means comprises a first flow tube and a second flow tube and that said Coriolis flowmeter further comprises:
brace bar means having a first end connected to said first flow tube and a second end connected to said second flow tube.
Another aspect is that the Coriolis flowmeter is characterized in that brace bar means are plastic.
Another aspect is that the Coriolis flowmeter is characterized in that said wetted flow path includes a plastic inlet flange and a plastic outlet flange each coupled to ends of said first flow tube and of said second flow tube.
Another aspect is that the Coriolis flowmeter is characterized in that said brace bar and said first flow tube and said second flow tube are enclosed within a plastic case.
Another aspect is that the Coriolis flowmeter is characterized in that said wetted flow path includes a plastic splitter manifold coupling said inlet flange to inlet portions of said first and second flow tubes and further includes a plastic combiner manifold coupling said outlet flange to outlet portions of said first and second flow tubes.
Another aspect is that the Coriolis flowmeter is characterized in that said first flow tube and said second flow tube are curved.
Another aspect is that the Coriolis flowmeter is characterized in that said wetted flow path includes:
a plastic inlet flange coupled to inlet ends of said first and second flow tubes; and a plastic outlet flange coupled to outlet ends of said first and second flow tubes.
Another aspect is that the Coriolis flowmeter is characterized in that said wetted flow path further comprises:
a plastic inlet manifold connecting said inlet flange to said inlet ends of said first and second flow tubes;
a plastic outlet manifold connecting said outlet flange to said outlet ends of said first and second flow tubes.
Another aspect is that the Coriolis flowmeter is characterized in that said brace bar and said first and second flow tubes and each of said manifolds are enclosed within a plastic case.
Another aspect is the Coriolis flowmeter further comprises:
a plastic case,
plastic coupling means that couples said case to said plastic flow tube means;
said flow tube means is plastic and positioned within said case and adapted to receive a material flow;
said driver vibrates said plastic flow tube means;
said pick off means is coupled to said plastic flow tube means for generating output signals representing Coriolis defections of said vibrating plastic flow tube means with material flow;
said output signals are applied to circuitry that generates information pertaining to said material flow.
Another aspect is that the Coriolis flowmeter is characterized in that said driver has a plastic bobbin coupled to said flow tube means; and
said pick off means having a plastic bobbin coupled to said flow tube means.
Another aspect is a method of fabricating structure of a Coriolis flowmeter including flow tube means; said method comprising the steps of:
forming a core defining a material flow path of said flow tube means by injecting a low melting point metal or soluble material into a cavity of a core mold with said cavity defining said material flow path;
placing said formed material flow path core into a cavity of a wrapper mold and closing said wrapper mold to form a cavity between the outer surface of said formed material flow path core and the interior surface of said cavity of said wrapper mold;
said cavity of said wrapper mold defines the outer surface of said flow tube means;
filling said cavity of said wrapper mold with plastic to form a molded plastic flow tube means that contains said formed material flow path core;
removing said molded plastic flow tube means containing said formed material flow path core from said wrapper mold; and
removing said formed material flow path core from said molded plastic flow tube means by raising the temperature of said molded plastic flow tube means above the melting point of said metal forming said material flow path core or by dissolving said molded formed material flow path core with a solvent.
Another aspect is that the method is characterized in that said cavity further has means that locates said formed material flow path core in said cavity of said wrapper mold;
Another aspect is that the method further includes the step of forming said core mold having said cavity that defines said material flow path of said flow tube means.
Another aspect is that the method further includes the step of forming a wrapper mold having a cavity that defines said outer surface of said flow tube means and further having said means that locates said formed material flow path core in said cavity of said wrapper mold.
Another aspect is that the method is characterized in that said flow tube means defines a pair of flow tubes;
the step of forming said core mold includes the step of forming said core mold so that said cavity of said core mold defines the material flow paths of said pair of flow tubes;
the step of forming said material flow path core includes the step of forming said material flow path core of said pair of flow tubes;
the step of filing said cavity of said wrapper mold with plastic includes the step of forming a molded plastic structure defining said pair of flow tubes each containing one of said material flow path cores.
Another aspect is that the method is characterized in that said fabricated Coriolis flowmeter structure further comprises:
a first brace bar coupling a first end of each of said pair of flow tubes to each other and a second brace bar coupling a second end of each of said flow tubes to each other;
characterized in that said step of forming a wrapper mold includes the step of forming a cavity in said wrapper mold that defines the outer surface of said fabricated Coriolis flowmeter structure including said first and second brace bars and said pair of flow tubes;
the step of filing said cavity of said wrapper mold with plastic includes the step of forming a plastic Coriolis flowmeter structure defining said pair of flow tubes and said brace bars and with said formed Coriolis flowmeter structure containing said formed material flow path core.
Another aspect is that the method is characterized in that said fabricated Coriolis flowmeter structure further comprises:
driver mounting elements and pick off mounting elements affixed to said first and second flow tubes;
said step of forming said wrapper mold includes the step of forming a cavity in said wrapper mold that defines the outer surface of said fabricated Coriolis flowmeter structure including said driver mounting elements and said pick off mounting elements;
characterized in that said wrapper mold has provisions for locating said formed core in said cavity of said wrapper mold;
characterized in that the step of filing said cavity of said wrapper mold with plastic includes the step of forming a molded plastic Coriolis flowmeter structure that contains said pair of flow tubes, said driver mounting element and said pick off mounting elements with said pair of flow tubes containing said formed material flow paths core.
Another aspect is that the method is characterized in that said fabricated flowmeter structure further comprises:
an inlet flange coupled to an inlet end of said flow tubes and an outlet flange coupled to an outlet end of said flow tubes;
characterized in that said step of forming a wrapper mold includes the step of forming having a cavity that defines the outer surface of said Coriolis flowmeter structure including said flow tubes, said first brace bar and said second brace bar, said inlet flange and said outlet flange:
the step of filing said cavity of said wrapper mold with plastic includes the step of forming a molded plastic Coriolis flowmeter structure that that defines the exterior surface of said flow tubes, said first and second brace bars and said inlet flange and said outlet flange with said plastic Coriolis flowmeter structure containing said formed material flow path core.
Another aspect is that the method is characterized in that said fabricated flowmeter structure further comprises:
an inlet manifold coupling said inlet flange to an inlet end of said flow tubes and an outlet manifold coupling said outlet flange to an outlet end of said flow tubes;
said step of forming a wrapper mold includes the step of forming having a cavity that defines the outer surface of said Coriolis flowmeter structure including said flow tubes, said first brace bar and said second brace bar, said inlet manifold and said outlet manifold, said inlet flange and said outlet flange;
the step of filing said cavity of said wrapper mold with plastic includes the step of forming a molded plastic Coriolis flowmeter structure that that defines the exterior surface of said flow tubes, said first and second brace bars and said inlet manifold and said outlet manifold, said inlet flange and said outlet flange with said plastic Coriolis flowmeter structure containing said formed flow path core.
Another aspect is that the method is characterized in that said fabricated Coriolis flowmeter structure comprises a flow tube and a concentric balance bar surrounding said flow tube;
the step of forming a core mold includes the steps of forming a first core mold having a cavity that defines the material flow path of said flow tube;
said step of forming a core mold further includes the step of forming a second core mold having a cavity that defines the space between the exterior surface of said flow tube and the interior surface of said balance bar;
the step of forming a core includes the steps of injecting low temperature metal or soluble material into said first core mold to form said material flow path core and further includes the step of injecting low temperature metal or soluble material into said second core mold to form a hollow balance bar core that defines said space between the exterior surface of said flow tube and said interior surface of said balance bar;
the step of forming said wrapper mold includes the steps of forming a cavity adapted to receive said formed material flow path core and said formed hollow balance bar core;
the step of placing includes the steps of placing said formed material flow path core into said wrapper mold cavity and placing said formed hollow balance bar core into said wrapper mold cavity so that said formed hollow balance bar core is concentric with said material flow path core;
the step of filling includes the step of filing said cavity of said wrapper mold with plastic to form a molded plastic Coriolis flowmeter structure that defines the outer surface of said flow tube and said concentric balance bar with said plastic Coriolis flowmeter structure containing said material flow path core and said hollow balance bar core.
Another aspect is that the method is characterized in that said fabricated Coriolis flowmeter structure further comprises:
a first brace bar coupling a first end of said balance bar to said flow tube and a second brace bar coupling second end of said balance bar to said flow tube;
said step of forming a wrapper mold includes the step of forming having a cavity in said wrapper mold that defines the outer surface of said Coriolis flowmeter structure including said flow tube and said balance bar as well as said first brace bar and said second brace bar;
the step of filing said cavity of said wrapper mold with plastic includes the step of forming a molded plastic Coriolis flowmeter structure that defines said flow tube and said concentric balance bar as well as said first and second brace bars and that contains said material flow path core and said hollow balance bar core.
Another aspect is that the method is characterized in that said fabricated flowmeter structure further comprises:
an inlet flange coupled to an inlet end of said flow tube and an outlet flange coupled to an outlet end of said flow tube;
characterized in that said step of forming a wrapper mold includes the step of forming having a cavity that defines the outer surface of said Coriolis flowmeter structure including said flow tube, said balance bar, said first brace bar and said second brace bar, said inlet flange and said outlet flange;
the step of filing said cavity of said wrapper mold with plastic includes the step of forming a molded plastic Coriolis flowmeter structure that that defines the exterior surface of said flow tube, said balance bar, said first and second brace bars and said inlet flange and said outlet flange with said plastic Coriolis flowmeter structure containing said formed flow path core and said hollow balance bar core.
Another aspect is that the method is characterized in that said fabricated Coriolis flowmeter structure further comprises:
driver mounting elements and pick off mounting elements affixed to said balance bar;
the step of forming said wrapper mold includes the step of forming a cavity in said wrapper mold that defines the outer surface of said Coriolis flowmeter structure including said flow tube, said balance bar, said brace bars, said inlet manifold and said outlet manifold, and said driver mounting elements and pick off mounting elements;
the step of filing said cavity of said wrapper mold with plastic includes the step of forming a molded plastic Coriolis flowmeter structure whose outer surface defines said flow tube, said balance bar, said brace bars, said driver mounting elements and pick off mounting elements on said balance bar, said inlet manifold and said outlet manifold and with said plastic Coriolis flowmeter structure containing said formed material flow path core and said hollow balance bar core.